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Chapter 28 The Spinal and Peripheral Motor System Copyright © 2014 Elsevier Inc. All rights reserved. FIGURE 28.1 Schematic diagram of two step cycles in human locomotion. Drawings at the top illustrate the position of the limbs during one cycle. Dark horizontal bars below the drawings show the timing of activity in flexor and extensor muscles of the right leg. The division of each step cycle into stance (extension) and swing (flexion) phases is indicated at the bottom of the figure. During walking, the stance phases of the two legs overlap, with both feet in contact with the ground. Copyright © 2014 Elsevier Inc. All rights reserved. FIGURE 28.2 Motor neuron from a neonatal mouse spinal cord that has been filled with biocytin through an intracellular electrode and visualized by immunocytochemistry. (A) The extensive dendritic arbor is directed into the dorsal, medial, and ventral gray matter of the spinal cord, where dendrites receive thousands of synapses from interneurons, sensory afferents, and descending pathways. (B) Double staining for a marker of GABA ergic inhibitory terminals (GAD65/67) shows some of the inhibitory synapses along the proximal dendrites and cell body. Copyright © 2014 Elsevier Inc. All rights reserved. FIGURE 28.3 Diagram of the structure of the muscle showing the relationship between the thick and thin filaments that form the sarcomere, the sarcoplasmic reticulum (SR), and the location of the ion channels of the motor axon, neuromuscular junction, and muscle membrane. With permission, modified from Cooper and Jan (1999) Copyright © 2014 Elsevier Inc. All rights reserved. FIGURE 28.4 The three motor unit types (S, FR, and FF) can be defined experimentally by measuring their contractile properties and fatigability. Panels A–C show recordings of muscle force, with insets in A showing recordings of motor neuron action potentials. Note different time and amplitude calibration scales for each of the motor units. (A) Single twitches produced by one action potential of the motor neuron. (B) Maximal force produced by repetitive stimulation of the motor neuron to produce an unfused tetanus. In addition to differences in maximal force, the “sag” property, a dropping off of tension during maintained stimulation, is seen in FR and FF units. (C) Fatigability is demonstrated by a drop in the tension produced by a single twitch after short periods of activation, as noted. Note that S units show little fatigue, whereas FF units fatigue within 30 s. Reproduced with permission from Burke, Levine, Tsairis, and Zajac (1973). Copyright © 2014 Elsevier Inc. All rights reserved. FIGURE 28.5 Structure and innervation of muscle spindles and Golgi tendon organs. The sensory innervation of the spindle is through primary (Ia) spindle afferents that innervate both nuclear bag and nuclear chain intrafusal muscle fibers and secondary (II) afferents that innervate nuclear chain fibers. Motor innervation of the spindle is supplied by static and dynamic γ-motor neurons and by β-motor neurons. Copyright © 2014 Elsevier Inc. All rights reserved. FIGURE 28.6 Basic circuitry underlying the knee-jerk stretch reflex. Ia spindle afferents from the quadriceps muscle make monosynaptic, excitatory connections on α-motor neurons that innervate the quadriceps. Copyright © 2014 Elsevier Inc. All rights reserved. FIGURE 28.7 Cross-sectional schematic of the spinal cord depicting on the right side the origin of interneuron and motor neuron populations defined in developmental studies using molecular markers for cell identification. The ventral interneurons (V), derived from progenitors (p), are marked as V0–V3 based on the distinct position along the dorso-ventral axis (0 most dorsal, 3 most ventral) of the ventral horn. The left side of the spinal cord depicts the general location of commissural interneurons (CINs) and the Rexed’s laminae. CINs are characterized based on the projection of their axons, as ascending, descending, or both. Copyright © 2014 Elsevier Inc. All rights reserved. FIGURE 28.8 Scheme for multijoint coordination within the leg using a mosaic of “unit CPGs.” Burst generators produce movement in one direction, extension or flexion, at the hip (HE, HF), knee (KE, KF), ankle (AE, AF), or foot (FE, FF), with a separate generator for the muscle that lifts the toes (EDB). Burst generators for flexion and extension at each joint are mutually inhibitory, forming a unit CPG that produces alternation. The strength of excitatory (triangles) and inhibitory (circles) connections between unit CPGs can vary to create a variety of different gaits. Reproduced with permission from Grillner (1985). Copyright © 2014 Elsevier Inc. All rights reserved. FIGURE 28.9 (A) Drawing of an isolated neonatal mouse spinal cord used in in vitro experiments. Suction electrodes are placed in the ventral lumbar (L) roots to record motor neuron activity (blue and red) or in the dorsal root (gray) to stimulate sensory afferents. Glass electrodes are used to record intracellularly from individual neurons in the spinal cord. (B) The image shows a motor neuron (MN) filled with a fluorescent dye and visualized with a microscope during intracellular recording. Sustained electrical stimulation of sensory afferents can evoke rhythmic activity in motor neurons (intracellular record) characterized by rhythmic membrane depolarization which reaches the threshold for action potentials. This stimulation also results in alternating rhythmic activity between the left and right side of the spinal cord [compare the activity in L1-left (red) and L1right (blue)]. It also produces alternating activity between the upper (L1) and lower (L6) segments of the spinal cord [compare the 2 red traces or the 2 blue traces], as highlighted in the dotted box. The duration of afferent stimulation is indicated by the graded bar. Copyright © 2014 Elsevier Inc. All rights reserved. FIGURE 28.10 Schematic model with some of the known candidate interneurons that are involved in the modulation of the mammalian locomotor CPG. Excitatory interneurons (red) with rhythmic activity include classes of commissural interneurons (CIN) and the interneurons identified by expression of Sim1, Chx10, and Hb9 transcription factors. Inhibitory interneurons (blue) that participate in termination of excitatory bursts include the Renshaw (Ren), the Ia interneuron (IaIN), and some commissural interneurons (CIN). Modified from Grillner (2006). Copyright © 2014 Elsevier Inc. All rights reserved. FIGURE 28.B1 Schematic diagram of a neuromuscular junction showing key components subserving neuromuscular transmission and the myasthenic disorders associated with these components. Copyright © 2014 Elsevier Inc. All rights reserved.